With the rise of terrorism there is a growing need for effective detectors for radioactive weapons of mass destruction, or materials used to shield their radiation form detection, e.g., high atomic weight elements. Three weapons of special concern are so-called “dirty bombs”, uranium-based atomic bombs, and plutonium-based atomic bombs. For example, dirty bombs include chemical explosives surrounded by radioactive materials to be dispersed upon detonation, contaminating the surroundings. Dirty bombs can be detected by their emitted radiation, gamma and bremsstrahlung radiation being the most common signatures. Uranium-based atomic bombs can in principle be identified by the signature gamma rays of 235U or 238U. The radiation flux from weapons-grade 235U is low, and therefore excellent efficiency and good energy resolution is desirable to distinguish 235U or 238U signature gamma rays from background gamma rays and from innocent sources. Plutonium-based atomic bombs can be detected by neutron emission. Neutron emitters are sufficiently rare that the detection of a neutron source several times above neutron background levels can be prima facie evidence for the presence of plutonium.
The detection of gamma rays and neutrons has a long history dating from their discoveries. Many topical books and monographs are available, for example, “Radiation Detection and Measurement”, Third Edition, 1999 by Glenn F. Knoll, Wiley Press, the entire teachings of which are incorporated herein by reference. Until recently, radiation detectors were used almost exclusively for benign commercial or research applications. Gamma ray devices with good efficiency and energy resolution have been available since NaI(Tl), the most widely used inorganic scintillator, was introduced in the late 1940's. There are now a number of inorganic and organic scintillators, as well as a number of semiconductor detectors, such as high-purity germanium, that are commercially available for detecting gamma rays of low and high energy in configurations adapted for a variety of applications. Light from the scintillators can be detected by an optical detector, e.g., photomultipliers, photodiodes, and charge-coupled devices (CCDs) and the like. However, these detectors cannot detect gamma ray sources shielded by a sufficient mass of a high Z material, e.g., lead, tungsten, and the like. Commercial neutron detectors also became available in the early 1960s. These relatively bulky devices detect thermal neutrons are typically detected with gas-proportional counters filled with either BF3 or 3He. High energy neutrons can typically be measured by plastic and liquid scintillators that detect the highly ionizing protons produced when the energetic neutrons collide elastically with the hydrogen nuclei. The presence of fast neutrons can also be determined by thermalizing, or moderating the speed of the neutrons with a hydrogenous material, and detecting the resulting thermal neutrons with efficient thermal neutron detectors. Plastic and liquid scintillator containing lithium or boron are examples of detectors that employ this method.